The theoretical physicist, author and futurist made the prediction during a Reddit AMA last week.
“Let me stick my neck out,” he wrote. “I personally feel that within this century, we will make contact with an alien civilization, by listening in on their radio communications.”
“But talking to them will be difficult, since they could be tens of light years away. So, in the meantime, we must decipher their language to understand their level of technology. Are they Type I, II, or III??? And what are their intentions. Are they expansive and aggressive, or peaceful.”
“Another possibility is that they land on the White House lawn and announce their existence. But I think that is unlikely, since we would be like forest animals to them, i.e. not worth communicating with.”
The ‘types’ he refers to are based on the Kardashev scale, which measures how advanced an alien civilization is likely to be based on the amount of energy they are able to harness and use.
A video of Kaku describing these categories in his own words can be viewed below.